In a mobile communication system, an OFDM scheme is under study. OFDM uses a low symbol rate per carrier, and therefore using OFDM alone makes a system resistant to multipaths. Introducing guard intervals makes the system even more resistant to multipaths. Guard intervals can be realized by copying the waveform of a posterior portion of symbol data to the beginning of the symbol data. In this way, it is possible to correctly receive signals for a delay signals having a length shorter than the guard interval length in an OFDM communication. However, since guard intervals do not have a role as data sections, the transmission rate improves when guard intervals are as short as possible.
On the other hand, a system which sends an OFDM signal with directivity added thereto is also under study. In the system which transmits data with directivity, a base station transmits a signal with directivity oriented in the direction of arrival of an incoming signal from a mobile station. When there is a plurality of directions of arrival, the base station transmits data in the direction in which data is received with maximum power from the mobile station or transmits data with a plurality of directivities in the respective directions. The receiving side can obtain a path diversity effect from such signals transmitted with a plurality of directivities, and can thereby obtain data of good quality with fewer errors.
However, large delay time differences are produced among arriving signals transmitted with a plurality of directivities through their respective paths, and therefore the delay time differences may exceed a range of guard intervals unless sufficiently long guard intervals are inserted in transmission data compared to a case where signals are transmitted with a single directivity. In order to prevent delay time differences from exceeding the guard interval range, there is a scheme under which data is transmitted with a guard interval having the same length as a maximum arrival time among arrival times for the respective directivities after data is transmitted until the other party of communication receives the data. According to this scheme, even when large delay time differences are produced among incoming signals in their respective paths, the delay time differences fall within the guard interval range, and therefore no interference is produced among symbols.
However, in the case of a conventional transmission apparatus and transmission method, when data is transmitted with a plurality of directivities, a guard interval having the same length as that of the maximum arrival time among arrival times of the respective directivities is inserted, it is always necessary to design guard intervals according to the maximum arrival time, which leads to a problem that guard intervals are longer compared to a system in which signals are transmitted with a single directivity.